Being new to this group and not knowing much I should probably not try to give you any advice. Especially when I have problem of my own I can’t fix. But here is what I think could be happening.
Maybe incomplete combustion. CO2 also produce some CO signal with mass 28 and 29. 13C relative abundance is much higher than 15N, therefore enriching your 15N values with very small amount. A few blank would easily confirm you have some carry over from incomplete combustion. It would make some sense since all you changed is a new combustion unit and your values are enriched. All other values would still be ok, since only the nitrogen (at the beginning of the run) is affected by the incomplete combustion signal carried over from the last sample. Just an idea….
We recently installed a Vario ISOTOPE Cube and overall it’s running well. It’s currently configured for CNS. I’ve been running our in-house standards to check the system out. Carbon values look fine and similar to what I was getting with the Eurovector. Sulfur is consistent, although I need to run some sulfur standards to confirm I’m getting close to the right values.
Nitrogen, though, is odd. The values for peptone, keratin and glycine are all 2-3 permil higher than expected and NIST 1547 and apple leaves are about 4-6 permil too heavy. It’s like I’ve got an enriched N contamination. Other than a new EA, nothing else has changed in the system. If anyone has an idea or suggestions, I would really appreciate it!
IRMS Laboratory Manager
Boston University Stable Isotope Laboratory
Department of Biology
5 Cummington Mall
Boston, MA 02215
"You are what you eat, plus a few permil!"